The invention relates to an improved soldering process system for monitoring critical process variables including time-on-connection, tip temperature and heat transferred to each connection.
The objective in high reliability soldering is to maintain a constant tip temperature profile for each of the connections soldered and a constant time on contact. This consistency in temperature and time on contact provides a steady thermal profile for the entire assembly. The ideal tip temperature profile for a soldering tool is a straight line indicating no tip temperature lost. This ideal tip temperature profile is rarely achieved due to the small thermal mass of the soldering iron tip. The illusive nature of maintaining the ideal temperature profile is why a process control is needed in the manual soldering process. A manual soldering process is difficult to control because hum,ms do not repeat themselves with the same consistency and repeatability of a machine. Most electronics manufacturing devices generate statistical process control (SPC) information for management and process engineers. However, manual soldering currently does not produce any or very little statistical information leaving management, process engineers, and soldering operators in the dark about the performance of the manual soldering process. Critical characteristics, such as the number of solders performed per board, the soldering iron temperature drop and the time on contact are not regulated. It is therefore the object of this present invention to develop a soldering system that enables the process engineer to monitor these critical process variables. This information can be used to point to problems or to show that the probability of problems in the manual soldering process are small.
In manual soldering process design, the critical process variables can be controlled for quality production. Operator variability in the soldering process and the electrical characteristics of the iron are controllable and directly affect the solder connection reliability. An Operator Variability Coefficient (OVC) identifies the relationship between the operator's actions and response of the iron tip temperature. By using correlation analysis, the OVC can be derived to provide information on the consistency and control in the manual soldering process. The results can be used to ensure the production of reliable solder connections. Correlation analysis quantifies the mount of correlation between the variation in operator time on the connection and the variation in the tip temperature. The value obtained from this analysis generates a value between 0 and 1, where 1 is perfect correlation and 0 is the complete noncorrelation. Therefore, at the end of an assembly, the lower the correlation coefficient, the more stable the temperature during the assembly soldering process. The operator and the soldering iron's thermal characteristics affect the OVC. As the operator time between connections becomes more consistent, the correlation between time and temperature decreases or as the thermal decay of the tip becomes less time dependent.
The present invention automates the process monitoring technique by using a computer integrated system which allows observation of a manually controlled process in a reasonable and cost effective time frame. The manual soldering process system monitors real-time tip temperature, temperature drop, time-on-contact, and heat transferred during each soldering operation and generates a displays plotting tip temperature over time, comparisons of smoothed data over raw data, and statistical process control graphs over user specified periods of time, such as a single day, week, or a year. A wide range of graphs can be produced including mean and standard deviations for time on connection, mean and standard deviations of curve characterization, initial and average temperatures, total time to solder board and longest time on connection. The user can adjust critical software variables such as the smoothing coefficient, the number of consistent temperature decays or recoveries to check and soldering iron and tip being used.
The present invention provides the process engineer immediate information on the level of consistency and control for the manually driven process. Each reworked or soldered assembly has a characteristic variability coefficient. This variability coefficient becomes more refined over time with some process history. If the variability coefficient is higher than normal on a particular assembly it indicates that either the iron operator or assemble material is not controlled. With additional troubleshooting, the process can be adjusted before product reliability is impaired.
The inventor knows of no other soldering process control monitors in the prior art that predict operator clean and soldering operations and measure time on connection, tip temperature and heat transfer to the each connection and produce statistical process control graphs. Related prior art is concentrated on maintaining a constant tip temperature and not with the collection and analysis of the manual soldering process control statistics. For example, a related patent, U.S. Pat. No. 5,062,564 issued to Urban, discloses a soldering system that collects soldering tip temperature data and rapidly responds to tip loading, distinguishing between dean and solder decays in the temperature profile, and maintains a steady tip temperature.